I think most of the concern arises from the fact that children, some of whom are physically or emotionally handicapped, are involved.
I deal with this issue every year in the context of using photographs of special needs children in the high school yearbook I sponsor.
I think a posted notice at the entrance to the venue that photography of the event is taking place with (I presume) the intent of publication for future promotion of the event would suffice to warn anyone who has an "issue" with being photographed.
I will concur with the previous poster that if it's a ticket selling event open to the public there is no "reasonable expectation of privacy" for anyone in attendance so releases should not be necessary.
As a personal observation, the special needs parents I have dealt with over the years - in and out of school - have been enthusiastic about having their children photographed in their activities.
Many if not most photographers avoid taking their pictures out of an honestly misplaced concern for their feelings which actually leaves them feeling left out. Consequently, they are most often delighted to be included.
As the risk of being a bit flip, the special needs parents who object to having their children and themselves photographed in such a setting generally have their kids locked away in the basement anyway.